Recently, resource and energy savings have been socially demanded. As a result, the development of so-called low fuel consumption tires has been eagerly carried out to reduce power loss. For this purpose, not only have tire structures been modified and the weight of tires been reduced, but also, almost all of the rubber compositions in all of the parts of the tire have been replaced with those yielding a lower energy loss. However, utilization of lower energy loss rubber compositions in all of the parts of the tire has resulted in very poor ride feeling of the tire.
Based upon consideration of the above-mentioned result and on the highly-advanced technology of lower energy consumption, it has been proposed to prepare a rubber composition having high energy loss in the sidewalls which will bring about an improvement in ride feeling. Such a composition having high energy loss can be prepared, for example, by (1) an increased addition of regular carbon black, (2) use of higher-quality carbon black or (3) an increased addition of softener. However, each of these approaches has unavoidable drawbacks.
An increased addition of regular carbon black means an increase in the modulus of elasticity of the rubber composition. Not only is it impossible by this means to improve the ride feeling but it also unfavorably lowers the flex resistance.
The use of higher-quality carbon black increases the storage modulus of elasticity E' of the rubber composition and this is not desirable from the standpoint of cut growth which starts from a small incision on the sidewall.
An increased addition of softener greatly lowers the weather resistance of the composition.